1. Field of the Invention
The present invention relates to an organic electroluminescent device utilizing an electroluminescent phenomenon produced in organic substances, more particularly to a device which is configured to interpose an organic luminescent medium-containing layer between a positive electrode and a negative electrode and designed to emit light when an electric field is applied thereto.
2. Description of Related Art
An organic electroluminescent (may be hereinafter referred to as EL) is formed of a thin film containing an organic fluorescent material interposed between positive and negative electrodes, and is such designed that a hole and an electron are injected into the thin film where they recombine to create an electron excited state, such as an exiton. As this excited state is deactivated, light emission occurs (by fluorescence, phosphorescence, delayed fluorescence, luminescent phenomena accompanying transport of energy, or the like). The organic EL device emits light utilizing this mechanism.
Characteristically, the organic EL device is capable of planar light emission with a high level of luminance ranging from 100 to 10,000 cd/m2 when the applied voltage is about 10 volts. Also, different emission hues, from blue to red, can be obtained by selecting the type of the organic fluorescent material used in the organic EL device.
The improvement in emission efficiency of the organic EL device can be achieved by increasing the efficiency of electron injection, and the use of low work function metals or their alloys for negative electrode materials have been attempted to increase the electron injection efficiency. In U.S. Pat. No. 4,885,211 and Japanese Patent Laying-Open No. Hei 2-15595, for example, a negative electrode material is disclosed which contains at least 50 atomic % of Mg and at least 0.1 atomic % of metal having a work function of at least 4.0 eV. Japanese Patent Laying-Open No. Hei 8-209120 discloses the use for a negative electrode of an alloy formed of 0.005-10% by mass of an alkaline metal and a second metal. Japanese Patent Laying-Open No. Hei 9-232079 discloses a negative electrode material formed of an alloy which contains, by a total amount, 0.5-5 atomic % of an alkaline metal or an alkaline earth metal having a work function of up to 2.9 eV. Japanese Patent Laying-Open No. Hei 10-12381 discloses the use of a ternary alloy for a negative electrode material, which contains 1-30 atomic % of a metal having a work function of at least 4.0 eV, 0.002-2.0 atomic % of Li, and the balance of Mg.
However, the conventional techniques such as described in the above-cited references use negative electrode materials containing extremely lower work function metals, i.e., metals having higher tendencies to release electrons. When exposed to moisture or oxygen present in the air, such materials readily undergo oxidation to result in the accelerated deterioration of the negative electrodes. This has led to such problems as luminance reduction, build-up of operating voltage, formation and expansion of nonradiative regions called xe2x80x9cdark spotsxe2x80x9d.
An object of the present invention is to provide an organic electroluminescent device which, due to its utilization of a specific negative electrode material, can exhibit high levels of emission efficiency and emissive luminance and suppress a luminance drop during a long-term operation.
In accordance with a first aspect of the present invention, an organic electroluminescent device has a luminescent material-containing layer interposed between a positive electrode and a negative electrode, and is designed to supply an electrical energy to the luminescent material that emits light upon receipt of the energy. The negative electrode characteristically contains at least one element, xe2x80x9cfxe2x80x9d, selected from elements having electronegativity values greater than that of calcium (Fauling electronegativity value=1.0) and equal to or less than that of vanadium (Pauling electronegativity value=1.6), and at least one element, xe2x80x9cpxe2x80x9d, selected from elements having electronegativity values equal to or greater than that of aluminum (Pauling electronegativity value=1.5).
Examples of useful xe2x80x9cfxe2x80x9d elements include Be (1.5), Ti (1.5), V (1.6), Cr (1.6), Mn (1.5), Zr (1.4), Nb (1.6), La (1.1), C (1.1-1.2), Pr (1.1-1.2), Nd (1.1-1.2), Sm (1.1-1,2), Gd (1.1-1.2), Tb (1.1-1.2), Dy (1.1-1.2), Ho (1.1-1.2), Er (1.1-1.2), Tm (1.1-1.2), Lu (1.1-1.3), Hf (1.3) and Ta (1.5), wherein numerical values given in parentheses represent publicly available Pauling electronegativity values.
Examples of useful xe2x80x9cpxe2x80x9d elements include H (2.1), B (2.0), C (2.5), N (3.0), O (3.5), F (4.0), Al (1.5), Si (1.8), P (2.1), S (2.5), Cl (3.0), Ga (1.6), Ge (1.8), As (2.0), Se (2.4), Br (2.8), In (1.7), Sb (1.9), Te (2.1), I (2.5), Ti (1.8), Zn (1.6), Cd (1.7) and Hg (1.9), wherein numerical values given in parentheses represent publicly available Pauling electronegativity values.
In accordance with a second aspect of the present invention, an organic electroluminescent device has a luminescent material-containing layer interposed between a positive electrode and a negative electrode, and is designed to supply an electrical energy to the luminescent material that emits light upon receipt of the energy. The negative electrode characteristically contains at least one element, xe2x80x9cfxe2x80x9d, selected from elements having electronegativity values greater than that of calcium (Pauling electronegativity value=1.0) and equal to or less than that of vanadium (Pauling electronegativity value=1.6), at least one element, xe2x80x9cpxe2x80x9d, selected from elements having electronegativity values equal to or greater than that of aluminum (Pauling electronegativity value=1.5), and at least one element, xe2x80x9cdxe2x80x9d, selected from elements having electronegativity values equal to or greater than any of those of iron (Pauling electronegativity value=1.6), cobalt (Pauling electronegativity value=1.6) and nickel (Pauling electronegativity value=1.6) and equal to or less than that of gold (Pauling electronegativity value=2.4), wherein the xe2x80x9cdxe2x80x9d element selected is the element that is excluded from the selection of the xe2x80x9cfxe2x80x9d or xe2x80x9cpxe2x80x9d element.
Examples of useful xe2x80x9cdxe2x80x9d elements include Re (1.9), Fe (1.8), Ru (2.2), Os (2.2), Co (1.8), Rh (2.2 greater than , Ir (2.2), Ni (1.8), Pd (2.2), Pt (2.2), Cu (1.9), Au (2.4), Hg (1.9), Tl (1.8), Si (1.8), Ge (1.8), P (2.1), As (2.0), Sb (1.9), Se (2.4) and Te (2.1), wherein numerical values given in parentheses indicate publicly available Pauling electronegativity values.
In a third aspect of the present invention, the element, xe2x80x9cpxe2x80x9d, as used in the aforementioned first and second aspects, is selected from elements having electronegativity values equal to or greater than that of aluminum (Pauling electronegativity value=1.5), less than that of carbon (Pauling electronegativity value=2.5), and less than that of iodine (Pauling electronegativity value=2.5).
It is preferably that those elements, xe2x80x9cfxe2x80x9d, xe2x80x9cpxe2x80x9d and xe2x80x9cdxe2x80x9d, are selected from different groups in the periodic table, respectively. A preferred element content of the negative electrode material is in the range of 0.1-10% by mass (more preferably in the range of 0.3-3% by mass) for the xe2x80x9cfxe2x80x9d element, in the range of 0.1-99.5% by mass for the xe2x80x9cpxe2x80x9d element, and in the range of 0-99.8% by mass for the xe2x80x9cdxe2x80x9d element. When the three elements, xe2x80x9cfxe2x80x9d, xe2x80x9cpxe2x80x9d and xe2x80x9cdxe2x80x9d, are all contained in the negative electrode material, it is preferred that a sum of the xe2x80x9cpxe2x80x9d and xe2x80x9cdxe2x80x9d element contents is not below 90% by mass.
In a fourth aspect of the present invention, the luminescent material-containing layer, as used in the first through third aspects, contains at least a host as a principal constituent and a fluorescent dopant. A ratio in molar mass of the dopant molecule to the host molecule (dopant/host) is generally in the range of 0.344-2.90, preferably in the range of 0.441-2.26.
In a fifth aspect of the present invention, the xe2x80x9cfxe2x80x9d is at least one element selected from elements which have electronegativity values greater than that of calcium (Pauling electronegativity value=1.0), and, less than that of zirconium (Pauling electronegativity value=1.4) and which, in the form of simple substance, have melting points higher than that of lithium (literature-listed melting point =180.5xc2x0 C.), and, equal to or lower than that of lutetium (literature-listed melting point=1,660xc2x0 C.). Specifically, the xe2x80x9cfxe2x80x9d is at least one element selected from Sc (1,540xc2x0 C.), Y (1,520xc2x0 C.), La (921xc2x0 C.), Ce (799xc2x0 C.), Pr (931xc2x0 C.), Nd (1,020xc2x0 C.), Sm (1,080xc2x0 C.), Eu (822xc2x0 C.), Gd (1,310xc2x0 C.), Tb (1,360xc2x0 C.), Dy (1,410xc2x0 C.), Ho (1,470xc2x0 C.), Er (1,530xc2x0 C.), Tm (1,550xc2x0 C.), Yb (819xc2x0 C.) and Lu (1,660xc2x0 C.), wherein numerical values given in parentheses represent melting points of their simple substances as listed in a literature.
In a sixth aspect of the present invention, the xe2x80x9cfxe2x80x9d is the element whose simple substance has a melting point of lower than that of Sc (1,540xc2x0 C.). Specifically, the xe2x80x9cfxe2x80x9d is at least one element selected from Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er and Yb. These xe2x80x9cfxe2x80x9d elements are particularly suitable when negative electrodes are formed by a resistance-heat or electron-beam vapor deposition technique.
In a seventh aspect of the present invention, the xe2x80x9cfxe2x80x9d is the element whose simple substance has a melting point of below 1,000xc2x0 C. Specifically, the aft is at least one element selected from La, Ce, Pr, Eu and Yb. These xe2x80x9cfxe2x80x9d elements are particularly suitable when negative electrodes are formed by a resistance-heat vapor deposition technique.
In an eighth aspect of the present invention, the xe2x80x9cfxe2x80x9d is the element whose simple substance has a boiling point equal to or lower than that of Ce or Lu, and, equal to or higher than that of Dy. Specifically, the xe2x80x9cfxe2x80x9d is at least one element selected from Sc (2,830xc2x0 C.), Y (3,300xc2x0 C.), Ce (3,400xc2x0 C.), Pr (3,000xc2x0 C.), Nd (3,100xc2x0 C.), Gd (3,300xc2x0 C.), Tb (3,100xc2x0 C.), Dy (2,560xc2x0 C.), Ho (2,690xc2x0 C), Er (2,860xc2x0 C) and Lu (3,400xc2x0 C), wherein numerical values given in parentheses represent boiling points of their simple substances as listed in a literature. The xe2x80x9cfxe2x80x9d element is selected preferably from lanthanum series elements, more preferably from cerium group elements with atomic numbers 57 through 62 (Laxcx9cSm). These xe2x80x9cfxe2x80x9d elements are suitable when negative electrodes are formed by a resistance-heat vapor deposition technique, and are particularly suitable when negative electrodes are sputter formed.
In a ninth aspect of the present invention, the xe2x80x9cfxe2x80x9d is the element whose simple substance has a boiling point equal to or lower than that of Tm. Specifically, the xe2x80x9cfxe2x80x9d is at least one element selected from Sm (1,790xc2x0 C.), Eu (1,600xc2x0 C.), Tm (1,950xc2x0 C.) and Yb (1,194xc2x0 C.), wherein numerical values given in parentheses represent melting points of their simple substances as listed in a literature. Preferably, the xe2x80x9cfxe2x80x9d is the element whose simple substance has a boiling point equal to or higher than that of Eu. These xe2x80x9cfxe2x80x9d elements are suitable when negative electrodes are formed by a resistance heat vapor deposition technique, and are particularly suitable when negative electrodes are formed by an electron-beam vapor deposition technique.
In a tenth aspect of the present invention, the xe2x80x9cfxe2x80x9d is the element whose simple substance has a metallic bond radius equal to or smaller than that of cerium, and, equal to or larger than that of thulium or lutetium. Specifically, the xe2x80x9cfxe2x80x9d is at least one element selected from Ce (0.183 nm), Pr (0.182 nm), Nd (0.181 nm), Sm (0.179 nm), Gd (0.179 nm), Tb (0.176 nm), Dy (0.175 nm), Ho (0.174 nm), Er (0.173 nm), Tm (0.172 nm) and Lu (0.172 nm), wherein numerical values given in parentheses indicate metallic bond radii,of their simple substances as listed in a literature.
In an eleventh aspect of the present invention, the xe2x80x9cpxe2x80x9d is at least one element selected from Zn, B, Al, In, Tl, Si, Ge, Sn, P, Sb, Bi, S, Se and Te.
In a twelfth aspect of the present invention, the xe2x80x9cdxe2x80x9d is the transition metal element whose simple substance has a melting point lower than that of Mo (2,620xc2x0 C.). Specifically, the xe2x80x9cdxe2x80x9d is at least one element selected from Fe (1,540xc2x0 C.), Ru (2,310xc2x0 C.), Co (1,490xc2x0 C.), Rh (1,970xc2x0 C.), Ir (2,410xc2x0 C.), Ni (1,450xc2x0 C.), Pd (1,550xc2x0 C.), Pt (1,770xc2x0 C.), Cu (1,083xc2x0 C.), Ag (962xc2x0 C.) and Au (1,064xc2x0 C.), wherein numerical values given in parentheses indicate melting points of their respective simple substances as listed in a literature,
In a thirteenth aspect of the present invention, the xe2x80x9cpxe2x80x9d element is Al, while the xe2x80x9cdxe2x80x9d is the element whose simple substance has a boiling point equal to or lower than that of Co, and, equal to or higher than that of Ag. Specifically, the xe2x80x9cdxe2x80x9d is at least one element selected from Co (2,870xc2x0 C.), Ni (2,730xc2x0 C.), Cu (2,570xc2x0 C.) and Ag (2,210xc2x0 C.), wherein numerical values given in parentheses indicate boiling points of their respective simple substances as listed in a literature.
Al, if used as the xe2x80x9cpxe2x80x9d element, melts and vaporizes at temperatures respectively lower than the melting point (660.4xc2x0 C.) and boiling point (2,470xc2x0 C.) of its simple substance. This effectively facilitates formation of negative electrodes by a vacuum vapor deposition technique. Also, Al, if existing in the form of a simple substance, has the following physical properties; electrical resistivity=2.655 xcexcxcexa9cm, thermal conductivity=237 W/mxc2x7K, Young""s modulus=68.3 GN/m2 and coefficient of linear expansion=0.237xc3x9710xe2x88x924/K. Accordingly, the Al-containing negative electrodes exhibit excellent electrical and thermal conductivities, as well as appropriate levels of mechanical strength.
In a fourteenth aspect of the present invention, the xe2x80x9cpxe2x80x9d element is Sb, while the xe2x80x9cdxe2x80x9d is the element whose simple substance has a thermal conductivity equal to or higher than that of Al. Specifically, the xe2x80x9cdxe2x80x9d is at least one element selected from Ag (427 W/mxc2x7K), Cu (398 W/mxc2x7K), Au (315 W/mxc2x7K) and Al (237 W/mxc2x7K), wherein numerical values given in parentheses indicate thermal conductivities of their respective simple substances as listed in a literature.
Sb, if used as the xe2x80x9cpxe2x80x9d element, melts and vaporizes at temperatures respectively lower than the melting point (630.7xc2x0 C.) and boiling point (1,750xc2x0 C.) of its simple substance. This effectively facilitates formation of negative electrodes by a vacuum vapor deposition technique. Also, Sb, if existing in the form of simple substance, has the following physical properties; Young""s modulus=77.9 GN/m2 and coefficient of linear expansion=0.172xc3x9710xe2x88x924/K (parallel to a c-axis) and 0.080xc3x9710xe2x88x924/K (perpendicular to a c-axis). By the same reason as applied to Al, the Sb-containing negative electrodes exhibit appropriate levels of mechanical strength. Sb, if in the form of simple substance, also has the following physical properties; electrical resistivity=39.6 xcexcxcexa9cm and thermal conductivity=24.3 W/mxc2x7K. Accordingly, when desired to obtain negative electrodes having excellent electrical conductivities and excellent heat dissipating properties through heat conduction, Sb, as the xe2x80x9cpxe2x80x9d element, may preferably be used in combination with the other element whose simple substance has the reduced electrical resistivity and the increased thermal conductivity. Examples of such elements for use in combination with Sb are below listed with their respective physical properties measured when in the form of simple substance; Ag (electrical resistivity 1.59 xcexcxcexa9cm, Young""s modulus=76 GN/m2 and coefficient of linear expansion=0.193xc3x9710xe2x88x924/K), Cu (electrical resistivity=1.67 xcexcxcexa9cm (20xc2x0 C.), Young""s modulus=110 GN/m2 and coefficient of linear expansion=0.162xc3x9710xe2x88x924/K) and Au (electrical resistivity 2.35 xcexcxcexa9cm, Young""s modulus=80 GN/m2 and coefficient of linear expansion=0.142xc3x9710xe2x88x924/K).
In a fifteenth aspect of the present invention, the xe2x80x9cpxe2x80x9d element is Bi, while the xe2x80x9cdxe2x80x9d is the metallic element whose simple substance has a thermal conductivity equal to or higher than that of Au. Specifically, the xe2x80x9cdxe2x80x9d is at least one element selected from Ag, Cu and Au.
Bi, if used as the xe2x80x9cpxe2x80x9d element, melts and vaporizes at temperatures respectively lower than the melting point (271.3xc2x0 C.) and boiling point (1,560xc2x0 C.) of its simple substance. This effectively facilitates formation of negative electrodes by a vacuum vapor deposition technique. Also, Bi, if existing in the form of simple substance, has the following physical properties; Young""s modulus=31.7 GN/m2, coefficient of linear expansion=0.162xc3x9710xe2x88x924/K (parallel to a c-axis) and 0.120xc3x9710xe2x88x924/K (perpendicular to a c-axis). By the same reason as applied to Al and Sb, the Bi-containing negative electrodes exhibit appropriate levels of mechanical strength. Bi, if in the form of simple substance, also has the following physical properties; electrical resistivity=107 xcexcxcexa9cm and thermal conductivity =9.15 W/mxc2x7K (perpendicular to a c-axis). Accordingly, when desired to obtain negative electrodes having excellent electrical conductivities and excellent heat dissipating properties via heat conduction, Bi, as the xe2x80x9cpxe2x80x9d element, may preferably be used in combination with the other element whose simple substance has the reduced electrical resistivity and the increased thermal conductivity compared to the xe2x80x9cpxe2x80x9d element.
In a sixteenth aspect of the present invention, a mean electronegativity value Eave, as calculated from weighting an electronegativity value of each element constituting a negative electrode by a proportion in number of its atoms present in the negative electrode, is in the range of 1.50-1.91, assuming that an electronegativity value of a lanthanoid element (Ln), such as Ce, is 1.15. The mean electronegativity value Eave is preferably in the range of 1.50-1.59 or 1.80-1.91.
In a seventeenth aspect of the present invention, when a flow of an DC current drives the device to emit light with a controlled luminance of 100 cd/m2, an emission efficiency, as calculated by dividing the luminance by a current density, is not below 10.0 cd/A.
In accordance with a eighteenth aspect of the present invention, an organic electroluminescent device has a luminescent material-containing layer interposed between a positive electrode and a negative electrode, and is designed to supply an electrical energy to the luminescent material that emits light upon receipt of the energy. The negative electrode contains xe2x80x9cfxe2x80x9d and xe2x80x9cpxe2x80x9d elements. The xe2x80x9cfxe2x80x9d is the element whose simple substance has a metallic bond radius equal to or larger than that of Ce, and, equal to or smaller than that of Eu or Yb. Specifically, the Aft is at least one element selected from La (0.187 nm), Ce (0.183 nm), Eu (0.198 nm) and Yb (0.194 nm), wherein numerical values given in parentheses indicate metallic bond radii of their simple substances as listed in a literature. The xe2x80x9cpxe2x80x9d is the element hose simple substance has a melting point equal to or lower than that of Al (melting point=660.4xc2x0 C.), and, equal to or higher than that of Sn (melting point=231.97xc2x0 C.), as well as having a modulus of elasticity intension, i.e., Young""s modulus equal to or higher than that of Sn, and, equal to or lower than that of Zn. Specifically, the xe2x80x9cpxe2x80x9d is at least one element selected from Zn (96.5 GN/m2), Al (68.3 GN/m2), Sn (41.4 GN/m2) and Sb (77.9 GN/m2), wherein numerical values given in parentheses indicate Young""s moduli of their simple substances as listed in a literature.
In an nineteenth aspect of the present invention, the xe2x80x9cfxe2x80x9d and xe2x80x9cpxe2x80x9d elements, as used in the aforementioned 18th aspect, are Ce and Al, respectively.
In accordance with a twentieth aspect of the present invention, the negative electrode, as used in the preceding 1st, 3rd through 11th and 16th through 19th aspects, includes a first layer closest to the luminescent material-containing layer, a second layer overlying the first layer and a third layer overlying the second layer. The first negative electrode layer is substantially formed from the xe2x80x9cfxe2x80x9d element, the second negative electrode layer from a mixture or compound of the xe2x80x9cfxe2x80x9d and xe2x80x9cpxe2x80x9d elements, and the third negative electrode layer from the xe2x80x9cpxe2x80x9d element.
In accordance with the twenty-first aspect of the present invention, the second negative electrode layer, as used in the aforementioned 20th aspect, has a composition gradient in its thickness direction such that toward its interface with the third negative electrode layer from its interface with the first negative electrode layer, its xe2x80x9cfxe2x80x9d element content decreases while its xe2x80x9cpxe2x80x9d element content increases.
Such a composition graded structure of the second negative electrode layer not only contributes to the improved interlaminar adhesion between the first and second negative electrode layers and between the second and third negative electrode layers, but also serves to relax a thermal shock due to a difference in thermal expansivity between the first and third negative electrode layers.
In accordance with the twenty-second aspect of the present invention, at least one of the first, second and third negative electrode layers, as respectively used in the 20th aspect of the present invention, contains an additional element different from the constituent element thereof.
A high level of emission efficiency is realized by the organic EL device according to the present invention. For example, when the organic EL device is driven by a flow of an DC current to emit light with a luminance of 100 cd/m2, it can exhibit an emission efficiency of not below 7.0 cd/A, further of not below 10.0 cd/A, as calculated by dividing the luminance by a current density. Furthermore, the organic EL device can be obtained which exhibits an emission efficiency of not below 5.0 lm/w, further of not below 10.0 lm/w, as calculated by dividing a luminous flux emitted therefrom by an input power applied thereto.
A high level of luminance is also realized by the organic EL device according to the present invention. For example, the organic EL device can be obtained which exhibits an emissive luminance L5V of not below 250 cd/m2, further of not below 500 cd/m21 when the applied voltage is 5 volts.
Also in accordance with the present invention, a luminance drop during a long-term operation can be suppressed. For example, the organic EL device can be obtained which, when its operation by an DC constant current is started from an initial luminance of 500 cd/m2 and continued until a current density converges to a constant value, exhibits a luminance ratio R500h, as calculated by a ratio of the initial luminance to a luminance after the lapse of 500 hours, of not below 10%, further of not below 50%.
A definition of electronegativity was proposed by Pauling and alternatively by Mulliken, and have been modified to date by their successors. The electronegativity values according to the Pauling scale is used throughout the specification of this application. However, this is not intended to exclude the use of other scales, such as the Mulliken electronegativity scale, as a selection reference of elements for use in the practice of the present invention, since they are approximately in line with each other. The electronegativity is a parameter as a measure of the power of a bonding atom to attract electrons to itself. The greater the difference in electronegativity between two atoms that form a bond, the more likely one atom attracts electrons to itself. This results in an increased ionic character in the bond. The electronegativity can also be used as a measure of indicating an electro-donating or electron-accepting property of an atom. The less electronegative atom has a greater electro-donating property, while the more electronegative atom has a stronger electro-accepting property. The atom having an intermediate electronegativity has an amphoteric property. Electronegativity values for various elements are scaled by Pauling or Mulliken. Although closely related to physical properties of elements, the electronegativity is not a physicochemical quantity such as mass or voltage, and thus carries no unit.
If the Mulliken approach is applied, the value of electronegativity (EN) of a given element can be determined from its ionization energy (IE) and electron affinity (EA) by using the following experimental equation;
xe2x80x83EN=(IE+EA)/(544 kJxc2x7molxe2x88x921)
Also, if the Pauling approach is applied, the difference xcex4EAB in electronegativity between the two elements A and B in a compound AB can be obtained from the following equation;
xcex4EAB=0.088[{Erealxe2x88x92(EAAxc2x7EBB)0.5}/(kJxc2x7molxe2x88x921)]0.5
where Ereal is a measured value for a bond energy of a A-B bond formed between the two elements A and B, and EAA and EBB are measured values for bond enegies of Axe2x80x94A and Bxe2x80x94B bonds, respectively.
For the compound AB, if the difference xcex4EAB in electronegativity between the elements A and B is zero, the A-B bond has a 100% pure covalent character. If an absolute value of xcex4EAB is about 1.7, the A-B bond has 50% ionic and 50% covalent characters. If an absolute value of xcex4EAB is 2.0, the A-B bond has about 63% ionic and about 37% covalent characters, showing a marked ionic character. The followings illustrate proportions of ionic bonding property that indicate bonding properties of vaious hologen halide molecules, as estimated from measurements of dipole moments that exist in those molecules, and an absolute value of electronegativity difference xcex4EHX between hydrogen (H) and a halogen element (X) together constituting each of those hydrogen halide molecules; hydrogen iodide (ioninc bonding property=4%, xcex4EHI=0.4), hydrogen bromide (ioninc bonding property=11%, xcex4EHBr=0.7) and hydrogen chloride (ioninc bonding property=19%, xcex4EHCl=0.9).
In the manner as stated above, the electronegativity can be used as an important indication when determining bonding states between similar or dissimilar elements.
The concept of electronegativity is intrinsic to an element. For example, diamond, graphite and amorphous carbon are allotropic forms of carbon. They have the same electronegativity value, regardless of their allotropic forms. It is however pointed out that they have work functions significantly different from each other.
Work function is an indicator as frequently used heretofore when selecting elements for constituting a negative electrode of an organic EL device. This work function is a physicochemical quantity that varies sensively depending upon the surface conditions of materials, and can be an useful indicator if a collection of measured values for individual materials is obtained. However, the use of work function, as determined for each single element, as an indicator for property prediction, such as of compounds or mixtures consisting of plural elements, gives results that are not considered to be very good. In the present invention, the electronegativity, a characteristic value intrinsic to an element, is used as a representative indicator when selecting elements for constituting a negative electrode. The electronegativity is a numerical representation of the power of an element atom to hold electrons using an integer or real number, and the present invention utilizes electronegativity values which indicate element-intrinsic preperties, as representative indicators when selecting elements for constituting a negative electrode.
Alkaline or alkaline earth metals have been used to form negative electrodes, for the purpose of improving the efficiency with which electrons are injected into an organic compound layer of an organic EL device. Examples of representing alkaline metals include Cs (0.7), Rb (0.8), K (0.8), Na (0.9) and Li (1.0), and examples of representing alkaline earth metals include Ba (0.9), Sr(1.0), Ca (1.0) and the like, wherein numerical values given in paratheses indicate electronegativity values. All the alakaline metals used, as well as most of the alkaline earth metals used, have electronegativity values of not exceeding 1.0. Since such metals having electronegativity values of not exceeding 1.0 hold electrons very weakly, negative electrodes formed therefrom are readily subjected to oxidation and thus unstable. Also, those metals tend to form water-soluble ionic materials. Accordingly, the exposure thereof to an atmosphere results in the increased occurrence for them to start liquefying from surfaces upon absorption of water vapor or moisture, called a deliquescent phenomenon, which is a problem. For example, Fr, Cs, Rb, K and Na, members of an alakline metal family, are readily oxidized by oxygen in the air to change to their oxides. Li reacts with nitrogen in the air to change to its nitride. Ra, Ba, Sr, Ca and Mg, members of an alkaline earth metal family, are readily oxidized by oxygen in the air to change to their oxides. Fr, Cs, Rb, K, Na, Li, Ba, Sr and Ca also react with a cold water or an atmospheric humid air to produce metal hydroxides and a hydrogen gas.
All the simple salts formed from alakaline metals are soluble in water. LiNO3 and NaNO3 are deliquescent, for example.
In the present invention, a negative electrode at least contains the xe2x80x9cfxe2x80x9d element selected from those having electronegativity values higher than that of calcium, and, equal to or lower than that of vanadium, instead of the aforementioned element having an electronegativity value of not exceeding 1.0. An atom of the xe2x80x9cfxe2x80x9d element has an appropriately low electron-holding power, so that efficient injection of electrons can be achieved from a negative electrode region containing the xe2x80x9cfxe2x80x9d element into an organic compound layer of an organic EL device. Also, the atom constituting the xe2x80x9cfxe2x80x9d element, if alone, is less subjected to oxidation, compared to those having electronegativity values of not exceeding 1.0. Also, when the xe2x80x9cfxe2x80x9d element is combined with the xe2x80x9cpxe2x80x9d element having an electronegativity value of not below 1.5 to form a mixture or compound, oxidation and following deterioration of the negative electrode formed from the mixture or compound can be suppressed to such an extent that its satisfactory performance is maintained. This is explained by the following reason: The atom of the xe2x80x9cpxe2x80x9d element having an electronegativity value of not below 1.5, preferably in the range of 1.5-2.4, has a higher electron-holding power and the atom itself is relatively less subjected to oxidation. Accordingly, the arrangement of the xe2x80x9cpxe2x80x9d atoms to surround the xe2x80x9cfxe2x80x9d atoms protects the xe2x80x9cfxe2x80x9d atoms from contacting oxidizing species, such as oxygen, nitrogen and moisture.
Where the xe2x80x9cfxe2x80x9d element having an electronegativity value higher than that of calcium, and, equal to or lower than that of vanadium, together with the xe2x80x9cpxe2x80x9d element having an electronegativity value equal to or higher than that of aluminium, are included in a negative electrode, a difference xcex4E in electronegativity between the xe2x80x9cfxe2x80x9d and xe2x80x9cpxe2x80x9d elements does not fall below 0.8, probably resulting in, the formation of the bond having a 15% or higher ionic character between the xe2x80x9cfxe2x80x9d and xe2x80x9cpxe2x80x9d atoms. Once such a bond has been formed, it becomes unlikely that the xe2x80x9cfxe2x80x9d atoms are further oxidized as by oxygen or moisture. It is thus expected that the xe2x80x9cfxe2x80x9d atoms maintain their predetermined state contemplated in the fabrication of the device. Also, selected combinations of the particular xe2x80x9cfxe2x80x9d and xe2x80x9cpxe2x80x9d elements result in preventing the reaction with atmospheric water vapor, i.e., result in the reduced occurrence of a deliquescent phenomenon.
Besides the aforementioned xe2x80x9cfxe2x80x9d and xe2x80x9cpxe2x80x9d elements, an additional element, xe2x80x9cdxe2x80x9d, having an electronegativity value equal to or higher than any of those of iron, cobalt and nickel, and, equal to or lower than that of gold may further be included in a negative electrode. The xe2x80x9cdxe2x80x9d element has an appropriately high electron-holding power, and its atom itself is very unlikely to be oxidized. Accordingly, the arrangement of the xe2x80x9cdxe2x80x9d atoms to surround the xe2x80x9cfxe2x80x9d atoms serves to protect the xe2x80x9cfxe2x80x9d atoms from contacting oxidizing species, such as oxygen, nitrogen and moisture. Also, since the xe2x80x9cdxe2x80x9d element having an electronegativity value in the range of 1.8-2.4 has a higher tendency to produce a compound or mixture which exhibits a good electric or/and thermal conductivity, a compound or mixture further containing the xe2x80x9cdxe2x80x9d element, besides the xe2x80x9cfxe2x80x9d and xe2x80x9cpxe2x80x9d elements, exhibit improved electrical or/and thermal conductivity.
Where the xe2x80x9cfxe2x80x9d, xe2x80x9cpxe2x80x9d and xe2x80x9cdxe2x80x9d elements are respectively selected from different groups in the periodic table, a compound or mixture containing these three different elements possibly forms a material having a more complex structure than when containing the xe2x80x9cfxe2x80x9d, xe2x80x9cpxe2x80x9d or xe2x80x9cdxe2x80x9d element alone, and exhibits new physical properties. Such new physical properties include the improved mechanical strength resulting from the change in bonding state of atoms in the material and the increased chemical stability, for example.
As aforestated as the fourth aspect of the present invention, the luminescent material-containing layer may at least contain a host, as a principal constituent, and a fluorescent dopant such that a ratio in molar mass of the dopant molecule to the host molecule (dopant/host) is generally in the range of 0.344-2.90, preferably in the range of 0.441-2.26. When such a composition is vapor deposited, those dopant and host molecules behave similarly in a vapor phase, reach a substrate while forming a nearly perefectly mixed molecular beam in the vapor phase, and change to a solid phase. This enables formation of the luminescent material-containing layer (luminescent layer) from the nearly ideal mixture of the dopant and host molecules. Therefore, the improvement in performance, particularly in emission efficiency, of the organic EL device can be achieved. Its service life can also be extended.